Electronic metal detector



Patented Apr. 24, 1951 um-"rec PAT o 2,550,607 ELE omnomc METAL :nn'rnoiron flrgm k sh nake ainsb gnma; 1 App'lication' February 13,'1\948,'Serial No 5:141

;.This invention .'.relates .generallywxto;metalzd- :Ltectingand moreparticularly to the methoidrand :apparatus for detecting metals building econstruction. y l

During the processofconstruction. of buildings gone-or more.contractorsuandlltheir workmen tare =.employed to construct parts "of the lbuilding :of which .they .are .particularly skilled and even though the plans require :the proper -locationaof metal columns, beams,x-pipe, conduit, electrical work is enclosed. It is the ordinary practice to refer to the plans and guess-Where the metal parts are and then destroy a portion oi -the esurface-of the building in-an' attempt-to find such metal-parts. I

The principal object of --'-this invention is t the provision of a portable electronic =apparatus=-designed to indicate the exactlocation eta-hidden 'metal part.

--Anther=- object is i the provision of a new and '-novel electronic circuit for indicating-the exact location'of ahidden metalpart.

Another obj ectis the provisionofan electronic detector for *locatingobjects such' as electrical outlets covered by plaster by showing a -maximum coupling indication when-centered over the "object.

Another object is the'provisionofan electronic detector assembledin-anenclosurepermitting the replacement of the vacuum tube--a-nd =batteries without disturbing the circuit.

Another object is the'provision of er-tuned electronic triode indicating circuit wherein the indication is obtained by opposed electronic flow.

r Other objects and :advantagesappear hereinafter in the following 'description a-nd claims.

A practical embodiment illustrating :the prin- -ciples of this inventionis disclosed the accomcpanying drawing wherein Fig. 1 is auview in :section ofathe apparatus :comprising' this invention;

Fig. 2 is a plan view ofc thezstructurezshown Lin'Fig. 1;

coming; (stirs-.183

:55 he:top;cr i strument pa elzofth 9 .25 [10, .the qcondcnser {32-1 zby suitable iclin a ndsar .for -.connecting1-th ;sents a case .orsboxim d {u o .th :$i.dewa1lsi candle?!,etheizel dew ills 4 m 'ld-it1 etQP15- {P119 bottom meme boxci ir mova l secur d t tthi end-and side wallsbrymean otthel crfiw -8- ividedwith an;;0nen nginvwhic .to r c iwath s di- 3128813 :Jcurrent. mi-lliamm ter 1 0, Which in 391 11 1 ;tic111ar-;circi1it shownlisarransed. he dath etota cor one-milliampere owineito t e-zsi ezcf 11. 7 triQQe arc and tne-circuim-employed. ;1I:rnmedi tclyradiacen iii/he mil-liammeter the.tepiisrrorid dwitha lableqresisterli controlled byi h k iqb 1L2. rncmediately adjacen the TQ iSI' O Ti iafloub ci switch ,1 3 iarrangedato ebe zoperatedby ,theswitch all the top 1s arran .ed msupnortet md :l5; byl;means;ofitheiinslll 11g p951 l=. zthe mountingsscrewsill. W iarrangedzto carryzithert tll m :tube i ns inherendear-t ans imfinti'fif .zprongs .foreconnectmgzthe elements :cf the tube-with the socket; 155- repnar tu lals includes'1'the-..Q0IIC nS oarran ed o .be con- -nectegdacross.thczzterminal zcf the mil-liaimm ter arra-n cd zto he onnecte sacross .the' terminals of .the:-;pick.upa Z athercondenser zzaiIL cries :tion to theipickupr-; coil;;:2;2. .s plays the use of a grid leak 24 3c sister; 25. I-Ifheswhole ofithisgapparatusliisiconsame toibecreplacedc without-.idisturbing;thecrest of the circuit. ".The slower :Qhamberfil] 10f :th box, 1. whichjs; enclosed byathezremovableiloottom isxalsoarrangcd toz'containsthe B:;battery:i3l andthe.nsbatterycfl-lwhiqh mfi lic ajrllfil eil DIME rcyid dwwithrl ad 1 ntll app ratus enc s n h mm mhe:

rl-=Referring -.-.tov;the circuit ;dia gram= of Eig. it ew qb notedtha th -tub tfl'i f th sfilemqm typehaving a cathode .33sandthe grid 34 nd the plate elementoranode;35. The double-pole switch -l 3:-.is r id qewitn t econtacts 35, 1 ,BI-arranged toropen the B?and.A battery piri es c ve y ill qmsit v etermina ccfitne ;Fig. #3 is a circuit;diagram employing a flla- :3 a y .31 -lbein nnecte through i ll :mentxcathode triode anda rectangular pickup .-coil;. .and

i Fig. Aisha circuit diagram supplying-a heated cathodepentode. anda circular, pickupccoil.

switch-point. 3.6,to theanode 35, andthe positive terminal ofsthe A battery .32 beinaconneeted .throu hith qsw t .zp intF3Lt the wire 38 ,that

is connected to one side of the cathode 33 and' ,Referringprincipallyto EigsIJendrZ, lvrepre- 455 .to 01181 end of.the resistor zd efl'he othengend c 3 of the resistor 25 is connected by means of the wire 40 to one side of the variable resistor The other side of the variable resistor is connected to the wire 4| which in turn is connected to one side of the milliammeter l0, one side of the condenser 20, and to the center tap 42 of the pickup coil 22. The other side of the milliammeter ID and the condenser 20 is connected by means of the wire 43 to the other side of the cathode 33 into the negative of the A battery 32. One side of the grid leak 24 is connected to the wire 43 and the other side is connected directly to the grid and to the condenser 23. The other side of the gridcondenser 23 is connected to the end 45 of the pickup coil 22 the opposite end 46 of which is connected to the negative side of the B battery 3|. The oppojsite sides of the condenser 2| are connected between the terminals of the pickup coil 22.

V, The circuit connection just described is energized by the closing of the switch points 36 and '31 of the switch I3 which creates an electronic fiow from the filament or cathode 33 to the grid and plate circuits. This electronic fiow is traced from the plate 35 through the switch 36, the B battery 3|, the terminal 46 of the pickup coil 22 to the center tap 42, the wire 4|, the meter l0, and return to the cathode 33. The electronic flow through the grid circuit may likewise be traced from the cathode 33 to the grid 34, the wire 44, the grid leak 24, to the cathode and also the wire 44, the condenser 23, the terminal 45 of the pickup coil 22 and return through the "mid-tap 42 of the pickup coil 22, the wire 4|, the meter Ill and return to the cathode 33. This electronic flow through the meter l by the tuned plate and grid circuits is opposed by a counter-electronic flow from the negative side of the A battery 32 through the meter In in the opposite direction, the wire 4|, the resistors and 25 in series, the wire 38, the switch point 31 and return'to the positive side of the A battery 32. The electronic flow through the meter 110 by reason of the tuned grid and anode circuits thereby provides a positive reading on the "zero current through the milliammeter ID, the currents flowing in opposite directions balancing one another thereby providing a zero reading on the meter. When the pickup coil is brought into close proximity to metal, either "magnetic or non-magnetic, the metal has the efiect of'decreasing the impedance of the pickup coil reducing'the flow of grid current and'per- 'mittinjg an increased electronic flow through the anode circuit. This increased electronic flow of "plate current provides a positive reading on the milliammeter. If the pickup coil of the detector is grought into close proximity of the metal the electronic fiow of the anode circuit is correspondingly increased causing the hand of the meter to accordingly register an increased flow of current and as thepickup coil is drawn away from the metal the electronic flow decreases and the hand of, the meter returns to zero.

The circuit illustrated in Fig.4 is the same as that of Fig. 3 except that the vacuum tube employed is a pentode connected as a triode and is provided with the cathode 50 heated by the heating element 51 that may be energized from any suitable source such as the battery 52. The screen grid 53 is connected to the anode and the suppressor grid 54 is connected to the oathode. In all other respects the circuit is the same as that shown in Fig. 3. Connecting the pentode as a triode as shown provides a more sensitive detecting circuit with a greater indication.

The pickup coil 41 is formed circular in place of rectangular in order to provide for the maximum coupling with a metal object of substantially the same perimetral shape. The terminals 42, 45 and 46 of the pickup coil may be in the form. of plugs arranged to be received by complementary sockets in the end wall 5 of the box I. Although only two'shapes of the pickup coil are shown it is desirable to employ the pickup coil in the shape substantially the same as the object to be located. The rectangular coil as shown in Figs. 1 and 2 is designed for use in 5 locating the rectangular metal outlet boxes of an electrical system that may have been covered by the plaster board and plaster in the finished room of a house or building. The rectangular pickup coil is substantially of the same physical dimension asthe rectangular outlet box. When the detector is moved along closely adjacent the plastered wall and it comes Within the close proximity of the outlet the hand of the meter will immediately register a high current flow. However, when the detector is moved around until the pickup coil is in substantial registration with the rectangular outlet box a maximum current reading will be indicated on the meter l0. Slight movement of the box in any direction will 4 decrease this current reading. Owing to the fact that the rectangular pickup coil is substantially the same dimension as that of the outlet box a pencil may be employed to trace the marginal edge of the end 5 of the box on the plaster or boarded surface of the wall. When the detector is removed the operator can cut the finished wall with safety and find the outlet box disposed in exact alignment with the pencil lines that he drew on the surface of the wall.

If .it is desired to locate a circular or hexa- .gonally shaped outlet box the circular pickup coil 4'! of Fig. 4 may be employed in place of the rectangular coil. Thejmpedance of these coils of course is substantially the same and will provide the same electronic flow giving the 55 same reading. r r n Although it is preferable to employ a pickup coil that has substantially the same shape as that of the object being searched, however anothershaped coil will indicate the presence of metal; It may be diificult to find the exact center'of the object, owing to the fact that configuration of the coil and the object is different. If the object does not exactly match the coil the current reading indicated on the milliammeter will be of less degree because the field of the coil will not be equally affected by the metal object being searched.

It is highly desirable to properly tune the grid 'andjanode' circuits of this device and proportion the rates of electronic flow through the meter so that the detector, when drawn several inches away irom the surfaceof the wall, will notregis'ter'the presence of themetallic object. 'Thisis advantageous to the operator asthere is no doubt in his mind that the maximum intensity of the field causing current flow registered by the meter is affected only when the pickup coil matches the object and then only when the pickup coil is in relatively close proximity to the object. In other words, other metallic objects have substantially little orno effect on the reading of the milliammeter unless it is in close proximity to a selected object thereby providing a high degree of accuracy in detecting the concealed metal.

The particular advantage of the removable false bottom 1 permits the one to replace the triode I8 or either one of the batteries 3| and 32 without otherwise disturbing the circuit. When the false bottom I is in position it protects the triode from injury.

I claim:

1. A detecting apparatus for locating a hidden metal object which comprises a vacuum tube having an anode, cathode and control grid, a pickup coil with an intermediate tap, a B battery, a connection from the anode through the B battery to one end of the pickup coil, a grid condenser, a connection from the grid through the grid condenser to the other end of said pickup coil, a grid leak connected from the grid to the cathode, a second condenser connected in multiple with the pickup coil, a current reading device connected between the cathode and the 6 intermediate tap of the pickup coil, an A battery, a circuit to connect the A battery to supply energy to the cathode and to supply through the current reading device a current flow in a direction opposite to the current fiow induced by the B battery, a variable resistor in said A battery circuit, and switch means having inde pendent interruptors to open the B and A battery circuits.

2. The structure of claim 1 which also includes a condenser, and a connection placing said condenser in multiple with said current reading device.

FRANK M. SHOEMAKER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PA'IEN'IS Number Name Date 1,126,027 Jullig Jan. 26, 1915 1,431,638 Dowling Oct. 10, 1922 2,018,080 Martienssen Oct. 22, 1935 2,048,591 Berry July 21, 1936 2,160,356 Fore May 30, 1939 2,220,070 Aiken Nov. 5, 1940 2,242,312 Machts May 20, 1941 2,412,782 Palmer Dec. 17, 1946 

